In an effort to reduce the amount of NOx in the exhaust gas of a gas turbine, inventors Wilkes and Hilt devised the dual stage, dual mode combustor which is disclosed in U.S. Pat. No. 4,292,801 issued Oct. 6, 1981 to a common assignee of the present invention, and incorporated herein by reference. In this patent, it is disclosed that the amount of exhaust NOx can be greatly reduced, as compared with a conventional single stage, single fuel nozzle combustor, if two combustion chambers are provided. The specific configuration as described in the above identified patent includes an annular array of primary nozzles each of which discharges fuel into the primary combustion chamber, and a central secondary nozzle which discharges fuel into the secondary combustion chamber. The secondary nozzle has an axial fuel delivery pipe surrounded at its discharge end by an air swirler which provides combustion air to the fuel nozzle discharge. Other components of the combustor include the combustion chamber liner, a venturi arranged in the secondary combustion chamber or zone, and the combustion chamber cap/centerbody.
The combustor is operated by first introducing fuel and air into the first or primary chamber for burning therein. Thereafter, the flow of fuel is shifted into the second chamber until burning in the first chamber terminates, followed by a reshifting of fuel distribution into the first chamber for mixing purposes, with burning occurring only in the second chamber. The combustion in the second chamber is rapidly quenched by the introduction of substantial amounts of dilution air into the downstream end of the second chamber to reduce the residence time of the products of combustion at NOx reducing temperatures thereby providing a motive force for the turbine section which is characterized by low amounts of NOx, carbon monoxide and unburned hydrocarbon emissions.
Further development in this area produced a two stage (diffusion/premixing) secondary fuel nozzle as described in commonly assigned U.S. Pat. No. 4,982,570. As described in the above identified patent, further reduction in the production of NOx may be achieved by altering the design of the central or secondary nozzle to operate as a diffusion piloted premixed nozzle. In operation, a relatively small amount of fuel is used to sustain a diffusion pilot, while a premix section of the nozzle provides additional fuel for ignition of the main fuel supply from the upstream primary nozzles.
It was subsequently discovered that high combustion dynamic pressure activity was present during the transfer to premixed operation. One method of suppressing the combustion dynamics is to use a two-stage (premixed/diffusion) gas only secondary fuel nozzle as described in commonly assigned copending application Ser. No. 07/680,073 (now allowed). The entirety of the '073 application is incorporated herein by reference.